During the postdoctoral stay at the Physics Department at the University of Cincinnati I have been involved in projects aimed to explore the optical properties of self-assembled quantum dots. In the group of Prof. Howard Jackson and Prof. Leigh Smith, co-supervising five graduate students I have been carrying various optical experiments on Cd(Mn)Te QDs.

The most important research achievements:
1. Zero-field optically induced magnetization in CdMnTe QDs.
2. Spin relaxation of excitons in CdTe QDs.
3. SIL-based mapping of single QDs.
4. Sub-millisecond spin memory of single magnetic QDs. (still to come)
5. Optically tunable exciton Zeeman splitting in a single CdTe QD. (still to come)

Other projects:
6. Exciton-phonon coupling in CdTe QDs.
7. Influence of annealing on the optical properties of QDs.
8. Morphology of CdTe QD structures. (still to come)
9. Spin and symmetry of single QDs. (still to come)
10. Properties of CdSe QDs grown by shadow mask epitaxy. (still to come)

The results obtained during these three years have been published in about 20 papers and will be (or actually are) substantial parts of at least three Ph.D. dissertations (Tak Gurung, Tuan Nguyen and Kapila Hewaparakrama).


1. Zero-field optically induced magnetization in CdMnTe QDs.

In the case of magnetically doped CdMnTe QDs, where fraction (~5%) of Cd atoms was substituted by Mn ions, we have observed strongly circularly polarized emission when tuning the polarized excitation laser into the inhomogenously broadened photoluminescence PL band. In addition, we find that the polarization of the emission is controlled by the excitation polarization. These effects are present at zero external magnetic field. An example of resonantly excited photoluminescence spectra of a large CdMnTe QDs ensemble is depicted in Fig. 1.

We attribute this novel effect to optically induced magnetization of Mn ions within quantum dots. We propose that spin-polarized excitons through the spin interaction force the ferromagnetic alignment of magnetic impurities (in other words, a ferromagnetic exciton magnetic polaron is formed). This is schematically shown in Fig. 2. Remarkably, the temperature dependence reveals that the polarization of the QD emission is very stable, it diminishes around 170K. We have also carried out systematic studies of this effect as a faunction of the excitation power, external magnetic field, and QDs' size.
Importantly, using solid immersion lens combined with slit-confocal microscopy we have also demonstrated this optically induced magnetization on a single quantum dot level. We find that in order to ferromagnetically align Mn ions confined to a single QD, the excitons need to be photoexcited through spin-conserving process, such as longitudinal optical phonon-assisted absorption. Example of two-dimensional images detected with a CCD camera for a given set of single CdMnTe QDs are shown in Fig. 3. Clearly, the intensity of single dot emissions detected in polarizations identical to the excitation is significantly higher than for counter-polarized configurations.

Publications:
S. Mackowski, T. Gurung, H.E. Jackson, L.M. Smith, J. Kossut, G. Karczewski, "Exciton-controlled Magnetization in Single Magnetic Quantum Dots", accepted for publication in the Applied Physics Letters (2005)
S. Mackowski, T. Gurung, H.E. Jackson, L.M. Smith, J. Kossut, G. Karczewski, "Optically Induced Magnetization of CdMnTe Quantum Dots", AIP Conference Proceedings 772, 1327 (2005)
T. Gurung, S. Mackowski, H.E. Jackson, L.M. Smith, J. Kossut, G. Karczewski, "Optical Studies of Zero-Field Magnetization of CdMnTe Quantum Dots: Influence of Average Size and Composition of Quantum Dots", Journal of Applied Physics 96, 7407 (2004)
S. Mackowski, T. Gurung, T.A. Nguyen, H.E. Jackson, L.M. Smith, G. Karczewski, J. Kossut, "Optically-induced Magnetization of
CdMnTe Self-Assembled Quantum Dots", Applied Physics Letters 84, 3337 (2004)
S. Mackowski, T. Gurung, T.A. Nguyen, K.P. Hewaparakrama, H.E. Jackson, L.M. Smith, J. Wrobel, K. Fronc, J. Kossut, G. Karczewski, "Optical Studies of Spin Relaxation in CdTe Self-Assembled Quantum Dots", physica status solidi c 1, 767 (2004)
S. Mackowski, T. Gurung, T.A. Nguyen, H.E. Jackson, L.M. Smith, J. Kossut, G. Karczewski, "Optically Controlled Magnetization of Zero-Dimensional Magnetic Polarons in CdMnTe Self-Assembled Quantum Dots", physica status solidi b 241, 656 (2004)


2. Spin relaxation of excitons in CdTe QDs.

A number of experimental methods, mainly based on time resolved spectroscopy, have been proposed to study exciton spin dynamics in quantum dots. We have developed a continuous wave experiment that allows to determine the exciton spin relaxation time. It is based on analyzing the polarization of QD emission when exciting quasi-resonant with polarized laser. At zero magnetic field we do not observe any net circular polarization of the emission (Fig. 4). Since more than 20% of all the dots in the ensemble feature negligible (<20meV) exchange splitting, we interpret this result as an evidence for extremely fast spin relaxation for QDs with degenerate exciton levels.

In clear contrast, when magnetic field is applied these exciton levels are split and, as displayed in Fig. 5, the excitons maintain their spin polarization for times much longer than the exciton recombination time (~300 ps).
Namely, the difference between the polarization degrees measured for s- and s+ polarized excitations depends only on the recombination time and the spin relaxation time of the excitons between two Zeeman-split energy levels. As an example,  Fig. 6 displays the experimental results obtained for as-grown CdTe QDs, both resonantly excited spectra at B=4T and the difference between the polarization degrees for both excitations. The points (experiment) are compared to calculation using the equation below. The estimated exciton spin relaxation time is 5 ns.
In order to further confirm this approach, we have also studied the exciton spin relaxation in annealed CdTe QDs, where the average size of QDs is expected to increase. Upon annealing (Fig. 7) we observe a strong reduction of the exciton spin relaxation, probably due to weakening of the electronic confinement, as for larger dots the energy levels of excited states (such as light-hole) which contribute to spin scattering are closer to the ground state.
A very important finding of this project is the insensitivity of the polarization of the exciton emission in single CdTe QDs and in the ensemble on the number of LO phonons contribution to the excitation. As shown in Fig. 8, the polarization of the QD emission is the same for QDs populated using 1-, 2- or even 3LO phonon assisted absorption. This indicates, that for QDs, LO phonon assisted absorption is a process that conserves the spin angular momentum of the excitons.

Publications:
S. Mackowski, T. Gurung, H.E. Jackson, L.M. Smith, W. Heiss, J. Kossut, G. Karczewski, "Sensitivity of Exciton Spin Relaxation in Quantum Dots to Confining Potential", Applied Physics Letters 86, 103101 (2005)
S. Mackowski, T. Gurung, T.A. Nguyen, H.E. Jackson, L.M. Smith, G. Karczewski, J. Kossut, M. Dobrowolska, J.K. Furdyna, "Exciton Spin Relaxation in Symmetric Self-Assembled Quantum Dots", AIP Conference Proceedings 772, 1357 (2005)
S. Mackowski, T.A. Nguyen, T. Gurung, K.P. Hewaparakrama, H.E. Jackson, L.M. Smith, J. Wrobel, K. Fronc, J. Kossut, G. Karczewski, "Exciton Spin Relaxation in Resonantly Excited CdTe/ZnTe Self-Assembled Quantum Dots", Physical Review B 70, 245312 (2004)
S. Mackowski, T. Gurung, T.A. Nguyen, K.P. Hewaparakrama, H.E. Jackson, L.M. Smith, J. Wrobel, K. Fronc, J. Kossut, G. Karczewski, "Optical Studies of Spin Relaxation in CdTe Self-Assembled Quantum Dots", physica status solidi c 1, 767 (2004)
S. Mackowski, T.A. Nguyen, H.E. Jackson, L.M. Smith, J. Kossut, G. Karczewski, "Exciton Spin Relaxation Time in Quantum Dots Measured by Continuous-Wave Photoluminescence Spectroscopy", Applied Physics Letters 83, 5524 (2003)
S. Mackowski, L.M. Smith, H.E. Jackson, W. Heiss, J. Kossut, G. Karczewski, "Optical Properties of Annealed Self-Assembled CdTe Quantum Dots", Applied Physics Letters 83, 254 (2003)


3. SIL-based mapping of single QDs.

Solid immersion lens (SIL), placed between the sample and a microscope objective increases the effective numerical aperture of the collection optics. The actual increase could be as large as the refractive index of SIL's material. We have developed a SIL-based optical experiment that enabled us to map single QDs with a spatial resolution down to 250nm. The schematics of the setup is shown in Fig. 9. We used a SIL made of LaSFN9 glass (n=1.83) and 50x microscope objective with NA of 0.5.

In the experiment, the sample was uniformly illuminated by a defocused laser. Then, emission of QDs within a 400 nm wide stripe was imaged directly onto the slit of the spectrometer and detected with a CCD detector in a form of a two-dimensional image. The vertical scale corresponds to the QD position along the slit and the horizontal scale represent the actual emission energy of the QD. An example of such a map is given in Fig. 10. By taking cross-sections along the spatial axis we determine the actual spatial resolution of our system to be ~500nm and ~300nm for a hemispherical and truncated SIL, respectively.
Full three-dimensional image (surface scan) was obtained by taking sequences of two dimensional images while moving the cryostat with the sample using piezoelectric actuators.
An example of a spatial QD emission map measured for CdTe QDs is presented in Fig. 11. The PL spectra extracted at two different spatial positions reveal symmetric and elongated QDs.
We have also applied this method to study the optical properties of single QDs in external magnetic field - from these experiments both the exciton g-factor and diamagnetic shift, which is proportional to the size of the exciton confined to the QD have been extracted.

Publications:
K.P. Hewaparakrama, S. Mackowski, H.E. Jackson, L.M. Smith, G. Karczewski, J. Kossut, "Photoluminescence Imaging of CdTe/ZnTe Self-Assembled Quantum Dots", AIP Conference Proceedings 772, 711 (2005)
K.P. Hewaparakrama, A. Wilson, S. Mackowski, H.E. Jackson, L.M. Smith, G. Karczewski, "Sub-Wavelength Multi-Channel Imaging Using a Solid Immersion Lens: Spectroscopy of Excitons in Single Quantum Dots", Applied Physics Letters 85, 5463 (2004)


6. Exciton-phonon coupling in CdTe QDs.

Photolumienscence excitation (PLE) spectra of single CdTe QDs feature two types of excitations. The first one, which is usualy several meV broad and has relatively low intensity (see Fig. 12) is attributed to excitation into a virtual state mediated by exciton-LO phonon interaction. The second one (Fig. 12) is in contrast much more intense and significantly narrowed (typically less than 1 meV). Most certainly it is associated with a direct excited state-ground state excitation.

Both processes (excited state-ground state transition and LO phonon - assisted absorption) are also visible in resonantly excited PL spectra of large QD ensembles. However, as displayed in Fig. 13 the behavior of phonon replicas is clearly different for CdTe QDs (small QDs with typical diameter of 3-5nm) than for CdSe QDs (large QDs with typical diameter 8-10nm). In the first case the 1LO replica is strogly enhanced when the excitation is at the blue edge of the PL spectrum, whereas in the second case this enhancement is observed much closer to the maximum of the inhomogenous PL band.
In order to get quantitative information about the strength of the exciton - LO phonon coupling in these structures we fit the resonantly excited PL spectra, as shown in Fig. 13. As a result, the intensities of LO phonon replicas and excited state-ground state emission plotted in Fig. 14 were obtained as a function of the excitation energy. In the case of CdTe QDs a clear narrow maximum of the 1 Lo phonon intensity appears in the high energy side of the non-resonantly excited PL band. We attribute this to strong enhancement of the exciton - LO phonon coupling for QDs characterized with small sizes. Indeed, for QDs with larger sizes (whether CdSe as-grown QDs and CdTe annealed QDs) the intensity profile of the 1 LO replica follows the shape of the non-resonant QD emission. The latter means that the exciton-LO phonon coupling is roughly uniform over the ensemble.

Publications:
T.A. Nguyen, S. Mackowski, H.E. Jackson, L.M. Smith, M. Dobrowolska, J. Furdyna, K. Fronc, J. Wrobel, J. Kossut, G. Karczewski, "Resonant Spectroscopy of II-VI Self-Assembled Quantum Dots: Excited States and Exciton-LO Phonon Coupling", Physical Review B 70, 125306 (2004)
T.A. Nguyen, S. Mackowski, H.E. Jackson, L.M. Smith, G. Karczewski, J. Kossut, M. Dobrowolska, J.K. Furdyna, W. Heiss, "Exciton-LO Phonon Interaction in II-VI Self-Assembled Quantum Dots", physica status solidi c 1, 937 (2004)
T.A. Nguyen, S. Mackowski, H. Rho, L.M. Robinson, H.E. Jackson, L.M. Smith, S. Lee, M. Dobrowolska, J. Furdyna, G. Karczewski, "Resonant and Non-Resonant Photoluminescence of CdSe/ZnSe and CdTe/ZnTe Self-Assembled Quantum Dots", Proceedings of the 26th ICPS, ed. A R Long and J H Davies, Edinburgh 2002, Institute of Physics Conference Series 171, H157 (2003)
T.A. Nguyen, S. Mackowski, H. Rho, H.E. Jackson, L.M. Smith, J. Wrobel, K. Fronc, J. Kossut, G. Karczewski, M. Dobrowolska, J. Furdyna, "Resonant Photoluminescence Spectroscopy of CdSe/ZnSe and CdTe/ZnTe Self-Assembled Quantum Dots", MRS Symposium Proceedings, 737, E9.2.1 (2003)


7. Influence of annealing on the optical properties of QDs.

We have studied the influence of rapid thermal annealing (RTA) on the optical and magnetic properties of self-assembled QDs. Similar pieces of samples were annealed fro 15 seconds in Argon atmosphere in temperatures ranging from 370C to 520C. In Fig. 15 we show low temperature micro-PL spectra obtained for samples containing CdTe QDs and CdMnTe QDs. In both these cases a strong blue-shift is observed, which is accompanied with narrowing of the inhomogeneous broadening of the PL emission.

In addtion, time-resolved experiments (Fig. 16) have revealed strong reduction of the exciton recombination time for annealed QD samples, which may be due to formation of non-radiative recombination centers as a result of annealing-induced interdiffusion. The reduction of emitting QDs in annealed samples is further confirmed by surprising observation of single dot emission lines when the sample annealed at 520C was excited with a laser spot of 30 microns in diameter (Fig. 16).

Yet another interesting behavior is depicted in Fig. 17, where we show the evolution of the linewidth of single CdMnTe QD emission for samples annealed at different temperatures. It is obvious that as the RTA temperature increases, the typical linewidth is strongly supressed. We ascribe this effect to increase of an average QD size and/or partial evaporation of Mn ions out of the QDs.

Publications:
T. Gurung, S. Mackowski, H.E. Jackson, L.M. Smith, W. Heiss, J. Kossut, G. Karczewski, "Tuning the Optical and Magnetic Properties of II-VI Quantum Dots by Post-Growth Rapid Thermal Annealing", physica status solidi b 241, 652 (2004)
S. Mackowski, H.E. Jackson, L.M. Smith, W. Heiss, J. Kossut, G. Karczewski, "Tuning the Properties of Magnetic CdMnTe Quantum Dots", Applied Physics Letters 83, 3575 (2003)
S. Mackowski, L.M. Smith, H.E. Jackson, W. Heiss, J. Kossut, G. Karczewski, "Optical Properties of Annealed Self-Assembled CdTe Quantum Dots", Applied Physics Letters 83, 254 (2003)
S. Mackowski, T.A. Nguyen, H.E. Jackson, L.M. Smith, G. Karczewski, J. Kossut, W. Heiss, "Optical Properties of Semimagnetic Quantum Dots", MRS Symposium Proceedings, 737, E9.1.1 (2003)